If you're going to grad school in physics or astronomy, you are going to have to take the physics GRE. It's hard, and to do well you will have to study. The review books are universally considered to be terrible, so you are better off with studying the practice tests in depth and using study guides from the internet. The links below should help you out.
ETS official site
University GRE sites
Review sheets from SPS member Isaac Storch:
tex source file
If you have review sheets and would like to share them - or if you edit/ improve upon Isaac's - please email sps(at)physics.ucsb.edu and we'll post them for others to use.